1. Field of the Invention
The present invention relates to a control apparatus for brushless or commutatorless d.c. motors.
2. Description of the Prior Art
In the prior art there is known a control apparatus for brushless or commutatorless d.c. motors, in which the rotational speed of a brushless d.c. motor is controlled in accordance with the duty cycle of a control pulse signal for controlling the exciting currents flowing through the exciting coils of the motor in the pattern of the control pulse signal. Such type of control apparatus has a plurality of switching transistors for controlling the driving voltage and current supplied pulsewise to the exciting coils of the motor, and the rate of the conducting time of the switching transistors is controlled in response to a control pulse signal, whereby the levels of the voltage and the current supplied to the exciting coils are regulated in accordance with the duty cycle of the control pulse signal.
In the aforesaid control apparatus, the switching speed of the switching transistors has to be fast for obtaining smooth rotation of the brushless d.c. motor. However, the power consumed by the switching transistors when they are changed in their switching state produces heat in an amount proportional to the switching speed of the switching transistors. Therefore, high speed switching operation of the switching transistors may cause the temperature of the switching transistors to increase, whereby the switching transistors may burn out.